Matrix type LCDs having multiple rows of striping electrodes (referred to as scanning electrodes or common electrodes) and multiple columns of electrodes (referred to signaling electrodes or segment electrodes) arranged perpendicular to the scanning electrodes have been widely used as means for displaying information in dot presentation.
A picture is displayed on such an LCD display by applying a scanning voltage to the respective scanning electrodes in turn and simultaneously applying a signaling voltage to the multiple signaling electrodes. Liquid crystal elements (LCES) are formed at the intersections of the scanning electrodes and the signaling electrodes one for each intersection.
Each of the LCEs is controlled to have a specific transparency determined by an applied effective voltage for a predetermined period of time (one frame period) required to scan all the scanning electrodes once. This scanning provides a desired picture on the display for each frame period.
Referring to FIG. 8, there is shown a circuit arrangement of a conventional display drive for an LCD. The display drive generates first through sixth output voltages V0, V1, V2, V3, V4, and V5, respectively, to be supplied to the LCD. It should be understood that voltages represent potentials with respect to the ground potential unless otherwise stated. The LCD includes a display panel, a scanning drive circuit for sequentially scanning the scanning electrodes, and a signaling drive circuit for applying a signal voltage to the respective signaling electrodes in synchronism with the scanning.
A step-up circuit CHP comprises of a charge pump circuit, which is adapted to receive a supply voltage Vcc from a battery and a clock signal clk and step up the voltage Vcc to a step-up supply voltage Vdd.
The supply voltage Vdd is supplied to a voltage amplifier A1 together with a reference voltage Vref to obtain a first bias voltage V0r by amplifying the reference voltage Vref by a predetermined factor. The first bias voltage V0r is then divided by resistors R0-R4 to obtain second through fifth bias voltages V1r-V4r, respectively.
The first through the fifth bias voltages V0r-V4r are respectively input to a first through a fifth buffer circuits B0-B4 operating at the supply voltage Vdd, each of which outputs the same voltage V0-V4 as the respective input voltage V0r-V4r. The sixth voltage V5 has the ground potential.
Of these output voltages V0-V5, the first, second, fifth, and sixth output voltages V0, V1, V4, and V5 are supplied to the scanning drive circuit of the LCD. The first, third, fourth, and sixth output voltages V0, V2, V3, and V5 are supplied to the signaling drive circuit. These output voltages are selected and used in synchronism with an alternation signal FR of the LCD. In what follows the alternation signal FR will be described for one frame period.
FIG. 9 illustrates typical waveforms of drive voltages applied to a particular set of scanning electrode COMj and signaling electrode SEGk of an LCD having n scanning electrodes and m signaling electrodes.
In odd numbered frames (for which FR being high (H)), scanning electrodes COM1-COMn are scanned to sequentially select one scanning electrode, COMj say, at a time. The selected scanning electrode COMMj is supplied with the first output voltage V0. The rest of the scanning electrodes COM1-COMn (excluding COMj) are supplied with the fifth output voltage V4. On the other hand, the signaling electrodes SEG1-SEGm are supplied with either the fourth voltage V3 or the sixth voltage V5 in accord with the display signal associated with the selected scanning electrode selected.
Similarly, in the even numbered frames (for which FR being low (L)), the scanning electrodes COM1-COMn are scanned to sequentially select one scanning electrode at a time. The selected electrode COMj, say, is supplied with the sixth voltage V5. The rest of the scanning electrodes COM1-COMn excluding COMj are supplied with the second output voltage V1. On the other hand, signaling electrodes SEG1-SEGm are supplied with either the first output voltage V0 or the third output voltage V2 in accord with the display signal associated with the selected scanning electrode selected.
In this way, under the alternation control of the display elements, a picture defined by the display signal is displayed on the LCD.
Each of the display elements of the LCD functions as a capacitive element. As a consequence, when the voltage of a signaling electrode changes, the voltage of the scanning electrode associated with the signaling electrode varies, exhibiting a noise voltage. This voltage variation causes crosstalks, resulting in degradation of the quality of the displayable picture displayed.
As a measure against such voltage variations, a power supply unit for use with an LCD drive is disclosed in a reference WO00/41028 (Reference 1). This unit comprises: two voltage-follower type differential amplifiers each receiving a pair of a first and a second voltages NV and PV, respectively; an N-type transistor output circuit driven by one of the two differential amplifiers; and a P-type transistor output circuit driven by the other one of the differential amplifiers.
This power supply unit is also provided with separate charging and discharging operational amplifiers for driving liquid crystal display elements. JPA H9-292596 (Reference 2) and JPA H9-203885 (Reference 3) disclose a power supply circuit for use in an LCD drive adapted to switch between two operational amplifiers at the timing of charging and discharging by means of a switching circuit and a timing circuit generating a timing pulse for the switching.
However, in the power supply unit of reference 1 the pair of voltages NV and PV to be supplied to the two differential amplifiers have different magnitudes, that is, there is an offset between them, so that the power supply unit has a dead band in which both of the differential amplifiers become inoperable. It is noted that the voltage at the output node of the output circuit is detected. Hence, the voltage variation (or noise) that has taken place on the display electrode is greatly damped by a selector (voltage selection switch) of the drive circuit before it is detected at the output node of the output circuit. For this reason, the voltage variation (noise) on the display electrode cannot be detected accurately.
In the output circuits of references 2 and 3, charging and discharging operational amplifiers are switched by a timing signal. Thus, additional circuit means for generating the timing signal is necessary. Further, these drive units cannot control the switching to suppress the voltage variation.
It is, therefore, an object of the present invention to provide a display drive unit suitable for driving, for example, a matrix type LCD, the drive unit adapted to detect the voltage near the display panel and including a first output circuit having enhanced capability of providing output current to bring up its output voltage and a second output circuit having enhanced capability of providing output current to bring down its output voltage and capable of switching between the two output circuits without any dead band, thereby reducing crosstalks in, and hence improving the picture quality of, the display panel.